The present invention relates generally to systems for digitally processing the pulses generated in detector systems in response to absorbed radiation and, more particularly, to processing such pulses in low cost, high resolution, high rate spectrometers for x-rays or gamma rays.
There is a need, in very high counting rate applications such as synchrotron radiation research, for improved x-ray spectrometers. In many of these applications it is desired to detect and count x-rays of one particular energy under conditions where these x-rays of interest are greatly outnumbered by x-rays of a different but nearby energy. A typical example would include X-ray Absorption Spectroscopy (XAS) of dilute metallo-protein solutions, where elastically scattered incident x-rays (noise events) greatly outnumber the fluorescence x-rays (signal events) from the metal atoms of interest. Since the x-ray spectrometer's total count rate capability is limited by energy resolution considerations, it spends most of its time processing noise events, which limits the acquisition rate of good signal events. Under these conditions it is advantageous to employ multiple detector systems to increase the good signal acquisition rate. Commercial spectrometers with 13 channels are now commonly sold and many researchers are considering systems with up to 100 channels. This approach is limited by several factors, including cost, lack of high count rate capability with pileup inspection, the lack of an energy resolved analysis of the spectrum seen by each detector, the practical difficulties associated with retuning the processing electronics for a large number of detector channels, and, often, the sheer bulk of the required electronics.
Cost is an important issue because of the large number of detector channels to be implemented. Typical instrumentation for a single detector channel using a high quality analog spectroscopy amplifier and energy spectrum analyzer ("multi-channel analyzer" or MCA) costs approximately $6,000. The cost of outfitting the desired 100 channels is thus prohibitively expensive for the great majority of researchers. Because of price and counting rate considerations, usually only a energy window analysis ("single channel analyzer" or SCA) is used, even for systems with only a few detectors.
The throughput, or maximum countrate capability, of energy analyzing spectrometers is usually set by the time it takes for the energy analyzer to process a pulse. During this time the system is "dead" and cannot accept other pulses. Common Wilkinson-type MCAs, particularly the low cost variety available as personal computer cards, can be quite slow, usually limiting count rates to less than 50,000 per second. Faster MCAs of comparable accuracy are available, but are much more expensive. Because a factor of 10 increase is desired for synchrotron applications, MCAs are not usually employed and the cheaper and faster windowing SCAs are used instead.
To inspect for pileup, the spectrometer must be able to detect the arrival times of the pulses coming from the preamplifier and then reject those that are closer together than the spectrometer's shaping time. If this is not done, such pulses are summed by the processing circuitry ("piling up"), and produce spectral distortions in the output. Because pileup occurs as the square of the input pulse rate, pileup inspection is a necessity when operating at the high count rates encountered in synchrotron experiments. Common commercial spectroscopy amplifiers are primarily designed for nuclear applications, however, and do not function effectively with x-rays below 10 keV. Typical inspection intervals are 500 to 600 ns, meaning that pulses arriving closer together than this cannot be distinguished. For the very high data rates encountered in synchrotron applications, a shorter inspection interval of 200 ns or less would be a distinct benefit.
To be properly carried out, a significant fraction of important synchrotron experiments also require energy analysis. These are typically experiments done using softer x-rays, in the region of 2000-4000 eV, where the energy resolution of even the best spectrometers is not adequate to fully resolve the signal energy of interest from the background energies. In these cases a simple SCA window cannot be set to accept only signal counts. Instead a full energy analysis is required and peak fitting is used to extract the signal peak from any nearby background peaks.
Spectrometer tuning is an important issue because each channel requires individual adjustment each time a new range of x-ray energies is to be studied. In conventional instruments this involves, for each channel, setting the amplifier's shaping time, coarse gain and fine gain and then adjusting the SCA's window to only accept counts in the energy range of interest. Accomplishing this requires disconnecting the amplifier and SCA from each other and using a separate calibration system (usually an oscilloscope, gated amplifier and MCA) to make the window setting. Then the amplifier and SCA are reconnected. The procedure is laborious, time consuming, and difficult to accomplish without errors, particularly when large numbers of channels are involved.
Spectrometer bulk also becomes an issue when many detector channels are required. The conventional electronics required for a 13 element detector array alone completely fill an electronics rack. Thus considerably higher density is required if 100 element arrays are to be practically implemented.
For these synchrotron applications, and many others as well, it would thus be advantageous to have a low cost, small volume spectrometry device capable of providing full energy analysis with good energy resolution at high count rates and be further capable of being interfaced to a computer system so that necessary tuning operations could be accomplished automatically by an appropriate program.